#include <stdio.h>
#include <string.h>

int main() {
    char string1[20] = "hello ";
    char string2[20] = "hello ";
    char string3[] = "world!";
    strncat(string1, string3, 3);
    printf("%s\n", string1);
    strncat(string2, string3, 6);
    printf("%s\n", string2);
    return 0;
}